The present invention relates to a method and apparatus for adapting slip threshold values for a drive slip and/or brake slip control system to the tires fitted to a motor vehicle.
In the generally known method, the adhesion coefficient utilized between the roadway and the driven vehicle wheels is deduced from dynamic data of the vehicle acquired by wheel speed sensors individually allocated to the driven and the non-driven vehicle wheels, namely at least the speed of the vehicle and the slip of the driven vehicle wheels while the vehicle is in a dynamically stable state of motion, i.e. straight-ahead travel and equality of speed of the driven vehicle wheels. The .mu. slip characteristic of the tires is deduced from the correlation between this utilized adhesion coefficient (.mu..sub.a) and the drive slip measured for this purpose. If the slip requirement of the tires required for the transfer of moment is greater or less than a value characteristic for a standard type of the tires, the response thresholds for the drive slip and/or the antiblocking control are raised or lowered.
According to this conventional method disclosed in DE 37 41 247 C1, a measuring window corresponding to a predetermined adhesion coefficient .mu..sub.o of predetermined mean amount of, for example, 0.4, is selected in the driving performance diagram characteristic of the vehicle which is equipped with the control devices. The driving performance diagram specifies the relationship between speed and achievable vehicle acceleration for the various transmission ratios, for example the measuring range which contains the achievable acceleration values in each case associated with speeds between 0 km/h and 100 km/h, typical values being around 2 m/s.sup.2 in the selected example. Whenever value pairs from this measuring window are reached, with the vehicle speed and its propulsive acceleration being obtained from output signals of wheel speed sensors characteristic of the rotational wheel speeds of the non-driven front wheels in accordance with level and/or frequency, the current value of the drive slip is determined from the rotational wheel speeds of the driven rear wheels which are also measured by these individually allocated wheel speed sensors. If it is found that this drive slip is greater than the drive slip .lambda..sub.o, compatible with the dynamic data of the vehicle, or associated with these for standard tires (e.g. summer tires fitted to the vehicle) with which drive slip the drive torque of the vehicle can be applied at the adhesion coefficient .mu..sub.o, the response thresholds of the drive slip and, in the case of the antiblocking control, of the brake slip are also raised or lowered proportionally to the measured difference. As a result, a point acquisition of the .mu. slip characteristic of the tires is achieved such that the slip requirement, which differs for winter and summer tires, is adequately taken into consideration for the application of drive and brake torques.
This known method quite effectively takes into consideration different .mu. slip characteristics of the tires used in each case, in the sense of an adaptation of the control characteristic both of the drive slip control and of the antiblocking control in each case in the sense of better capability for utilizing drive moments provided in the drive train or braking forces which can be deployed by the brake system. It still, however, has at least the following disadvantages, essentially because of the restriction of the usable data to a measuring window which is allocated to a particular adhesion coefficient .mu..sub.o :
1. In a situation where the drive slip is acquired during steep uphill or downhill travel, the measurement has unavoidable errors which can lead to an inadequate raising or lowering of the response thresholds. PA1 2. Due to the fact that the data acquisition is limited to the comparatively narrow measuring window, a check of the usable adhesion coefficients and a corresponding change of the slip thresholds occur relatively infrequently so that, in practice, an adhesion-dependent change in the response thresholds is not possible. An updating of the response thresholds is inevitably very sluggish. PA1 1. Rotational speed u of the vehicle engine PA1 2. Mass delivery rate of the combustion air quantity supplied to the engine and/or fuel quantity supplied to the engine PA1 3. Wheel speeds of the driven and of the non-driven vehicle wheels and PA1 4. Axle load L.sub.HA at the driven vehicle wheels, from which is determined the adhesion coefficient (.mu..sub.a) instantaneously utilized in driving operation, from the relationship EQU .mu..sub.a =F.sub.z /L.sub.HA
It is therefore an object of the present invention to improve such method and provide an apparatus for effecting the improved method with the aim that the adaptation of the slip thresholds occurs independently of the topographic conditions to meet the requirements of each situation and that the drive or brake slip threshold values of the two control types can be updated more rapidly.
With respect to the method, this object has been achieved by determining the drive torque provided at the driven vehicle wheels from fuel supply and/or air supply, engine speed and total transmission ratio of the drive train. The adhesion coefficient (.mu..sub.a) utilized is determined, with predetermined or measured normal, axial load force at the driven wheels in accordance with the relationship EQU .mu..sub.a =F.sub.z /L.sub.HA
in which F.sub.z is the applicable traction force of the vehicle drive train which is given by the relationship EQU F.sub.z =M.sub.z /r.sub.dyn
in which r.sub.dyn is the dynamic rolling radius of the driven vehicle wheels and M.sub.z is the propulsive torque of the drive train applied via the roadway. From the correlation of the determined adhesion coefficient .mu..sub.a with the measured drive slip, the .mu. slip characteristic of the tires is determined and, corresponding thereto, the response slip thresholds .lambda..sub.A and .lambda..sub.B are set for the drive slip and antiblocking control.
According to other aspects of the present invention, the output torque of the vehicle engine is determined from the mass delivery rate of the combustion air supplied to the combustion chambers of the engine, and the engine torque is determined from the fuel quantity supplied to the engine.
Accordingly, the drive torque provided at the driven vehicle wheels is determined from a family of engine characteristics which reproduces the relationship between fuel supply and/or air supply and the engine speed, and the overall transmission ratio of the drive train. Thereby, the traction force K.sub.z effective at the driven vehicle wheels can be determined with a predetermined normal force, either assumed to correspond to a mean value or, if appropriate measuring devices are provided (such as, for example, sensors responding to the spring deflection state of the vehicle wheels or pressure transducers generating signals characteristic of the pressure in a level-regulating system), a measured normal force, which traction force satisfies the relation EQU K.sub.z =.mu..sub.a .multidot.K.sub.N
in which .mu..sub.a designates the adhesion coefficient utilized and K.sub.N designates the effective normal force, and therefore the usable adhesion coefficient .mu..sub.a can be determined in accordance with the relation EQU .mu..sub.a =K.sub.z /K.sub.N .multidot.
This relation can be continuously evaluated by conventional technical apparatus also within the scope of a device provided for carrying out the method of the present invention.
Thus, the adhesion coefficient .mu..sub.a which can be utilized in each case is continuously determined and, at the same time, the .mu. slip characteristic of the tires is determined from the value of the drive slip which can also be continuously acquired. In connection therewith, the situations of the vehicle suitable for this drive slip are recognized from the fact that the front wheels, assumed to be not driven, have the same wheel speed, which is evaluated as an indication of straight-ahead travel of the vehicle, and also the driven rear wheels have identical, though greater, wheel speeds, from which the drive slip is determined by comparison with the wheel speeds of the front wheels. From the equality of the drive slip values of the driven rear wheels, a stable dynamic characteristic of the vehicle is deduced.
Among the results of the method according to the present invention are that the amount of the usable adhesion, which can be detected from the drive torque provided at the driven vehicle wheels and the drive slip values correlated therewith, is independent of the roadway topography and therefore meets the requirements of all situations. Another advantage is that the updating of the adhesion coefficients and required slip values which can be detected by this method is much closer to real time because it occurs more frequently than in the conventional method which is restricted to the acquisition of corresponding value pairs within a limited measuring window range.
As a result, a large number of errors in the control strategy are avoided which are otherwise possible such as, for example a premature response of the drive slip control during uphill driving. Another erroneous control strategy avoided is the faulty response of the control in cases in which the .mu. slip characteristic, particularly of the driven vehicle wheels, changes due to the operation of the vehicle, for example due to the fact that the tires of the driven rear wheels become very hot due to the manner of driving and, as a result, the slip requirement for transferring a predetermined traction force is increased.
The drive torque provided in the driven vehicle wheels can be determined via the family of engine characteristics from the volumetric efficiency of the combustion air supplied to the combustion chambers of the engine and, alternatively or in combination therewith, from the fuel quantity supplied to the engine, that is to say data which are in any case provided for processing in electronic control devices in current engine control systems. As a result in the final analysis, an optimum utilization of the fuel and/or also the prerequisites for catalytic minimization of the pollutant emission is to be achieved, and can therefore also be advantageously utilized for determining the characteristic tire values.
It is also possible with the present invention to detect the drive torque provided at the driven vehicle wheels via force or torque sensors arranged in the drive train, by means of which, for example, twisting deformations of elements of the drive train, particularly of the universal shaft, and/or forces acting in the joints themselves can be detected.
Due to the fact that it also enables low adhesion coefficients to be detected, the method according to the present invention also offers the advantage of realizing adequate tracking of the response thresholds of the respective control types within the range of extremely low adhesion coefficients of, for example, 0.3 and less. As a result, a sensitive control characteristic even with extremely poor road conditions is achieved.
To this extent, control algorithms can be used, for example in the range of such low adhesion coefficients, which are particularly suitable for a dynamically stable characteristic of the vehicle in the low .mu. range.
A device for achieving the method according to the present invention is provided in a road vehicle which is equipped at least with a drive slip control device and preferably also with an antiblocking system. The regulation-oriented control such system or systems is provided by an electronic control unit which generates from processing output signals of wheel speed sensor, which are characteristic in level and/or frequency of the wheel speeds of the driven and the non-driven vehicle wheels, the required drive signals for the drive slip control device and/or the antiblocking system. As a result, in the case of a drive slip control, a vehicle wheel tending to spin is decelerated again by activating its wheel brake and, if necessary, the engine torque is reduced. In the case of anti-blocking control, the braking forces are reduced when threshold values of the brake slip .lambda..sub.b and/or of the wheel decelerations are exceeded. In addition to the electronic control unit of the slip control systems, an electronic processing unit receives inputs in the form of:
in which F.sub.z is the applicable traction force of the drive train of the vehicle which, in turn, is given by the relationship EQU F.sub.z =M.sub.z /r.sub.dyn
in which r.sub.dyn is the dynamic rolling radius of the driven vehicle wheels and M.sub.z the propulsive torque of the drive train, which is applied via the roadway and which, in turn, is determined from the relationship EQU M.sub.z =M.sub.mot .multidot.i.sub.Ges .multidot..eta..sub.A
in which i.sub.Ges is the overall transmission ratio of the drive train, .eta. is the total mechanical efficiency of the drive train of the vehicle, and M.sub.mot is the output torque of the engine which, in turn, is determined from the relationship EQU M.sub.mot =p.sub.me .multidot.V.sub.M /4.pi.
in which V.sub.M is the total displacement of the vehicle engine and p.sub.me is the mean combustion pressure in the engine which is determined from a family of curves characteristic of the vehicle engine.
The advantage of this apparatus can be seen from the fact that the determination of the engine torque from the mean effective combustion pressure and the engine displacement can be used for virtually al conventional vehicle types without being dependent on other special characteristics of their engine curves. This is of considerable advantage with respect to a virtually universal applicability of the device.
According to another aspect of the invention, a measurement variable suitable for determining the engine torque can be detected by the output signal of a conventional temperature-sensitive air mass sensor. If a device for detecting the loading state of the vehicle is also provided, this can also be used for detecting slopes or inclinations of the roadway and for an appropriate change in the response thresholds of the control types when driving uphill and downhill. Of course, the adaptation of known electronic control units of the respective control device(s) required for this are well within the skill of those in this art.
Moreover, a particularly simple and fast detection of a quantity significant for the variation of the tire characteristic can be provided by an electronic computer stage which continuously generates an electrical output signal characteristic of the adhesion coefficient (.mu..sub.a) utilized. A comparison stage which continuously generates an output signal characteristic of the slip (.lambda..sub.a) of the driven vehicle wheels and an evaluating unit synchronized with the electronic computer stage and the comparison stage are also provided. The evaluating unit forms from a multiplicity of mutually correlated .mu..sub.a, .lambda..sub.a value pairs a continuous sliding mean of the quotient .mu..sub.a /.lambda..sub.a as a quantity characteristic of the tires.